In a battery can of a secondary battery such as a lithium ion secondary battery, an electric generating element is housed, and an electrolyte is injected. The battery can is sealed by a battery lid. The electrolyte is injected into the battery can from an opening portion for liquid injection (hereinbelow referred to as a liquid injecting portion) provided in the battery lid, and after injection, the liquid injecting portion is sealed by a sealing plug. The sealing plug is inserted into the liquid injecting portion and is normally connected with the liquid injecting portion by means of laser beam welding or the like (PTL 1).